1. Field of the Invention
This invention relates to achieving greyscale in liquid crystal devices, in particular in multi-stable or bistable liquid crystal devices.
2. Description of the Prior Art
Bistable displays are inherently digital in nature, i.e. either the pixel is in one state or the other. However for displaying images it is preferable to have a level of contrast or greyscale for the image. Indeed an essential part of producing colour displays is the achievement of sufficient greyscale. For example, achievement of 4096 colours requires three separately coloured sub-pixels each capable of 16 distinct transmission or reflection levels.
Various mechanisms for achieving greyscale are known. Full colour bistable ferroelectric liquid crystal displays are known (N. Itoh et al. “17” Video-rate Full colour FLCD”, Proc. 5th International Displays Workshops, Kobe, Japan, pp205-208 (1998). Here 256 greys were achieved using a combination of spatial dither and temporal dither.
Spatial dither uses spatial subdivision to latch varying amounts of the pixel into each bistable state. Temporal dither divides the frame into sub-divisions each of which can be used to display a different image. Temporal dither however requires fast operation and also requires constant update, reducing the usefulness of bistable displays as low power devices. A high level of spatial dither is costly, both in terms of the additional electronic drivers needed, and the reduced etching yield for the least significant (i.e. smallest) electrodes.
Another approach is to generate greyscale through analogue levels. This is done using partial latching of the pixel. After blanking the pixel into one stable state an intermediate voltage level is applied. The applied voltage is insufficient to latch all of the pixel but nucleates domains of the opposite stable state and forms a random mixture of domains. Varying the applied signal can case the number and size of the domains to change leading to a continuos change in the transmission or reflection of the pixel. This approach is often used for bistable cholesteric liquid crystal devices X-Y. Huang et al. “Gray scale of bistable reflective cholesteric displays”, Proc SID XLX, LP. 1, pp810-813 (1998). However use of analogue levels in this way is dependent on the applied voltage, cell gap and temperature. Numerous variations need to be considered, including local alignment or temperature differences within the panel, transmission line losses associated with long thin electrodes, differences between drivers—either random or due to operating temperature—changes of cell gap associated with the flatness of the glass, or variation of the domain nucleation sites. Any of these variations will cause some change in the transmission or reflection from the pixel. This is shown in FIG. 1 where slight variations across a cell, such as ΔV lead to relatively large transmission errors ΔT. The total number of greys that can be achieved is related to the acceptable change in transmission caused by the variations, which is in turn related to the gradient of the latching characteristic. Attempts to widen the partial latch width to increase the number of analogue levels that may be achieved often results in an increased number of manufacturing steps.
To prevent overlapping grey levels the display tolerances can become very tight. Considering cell gap variations alone, for a cholesteric device such as described in P. Slikkerveer et al. “A fully flexible cholesteric LC matrix display”, Proc SID XXXIII, 5.2, pp27-29 (2002) sixteen analogue levels require cholesteric devices to be produced with tolerances of ±1 nm. Bistable Twisted Nematic devices, such as those of Tanaka et al.[“A Bistable Twisted nematic (BTN) LCD driven by passive-matrix addressing”, Proceedings of 15th IDRC, Hamamatsu, Japan, pp 259-262 (1995)] rely on obtaining the correct ratio of cell gap and helical pitch and therefore need tighter tolerances still to achieve reliable analogue greyscale.
U.S. Pat. No. 6, 094, 187 describes a ferroelectric liquid crystal device wherein greyscale is achieved by a combination of dither, either spatial or temporal, with the use of analogue levels. The pixel is divided into a number of bits which may be either spatial or temporal or both. At least two of the bits are addressed with more than two grey levels, i.e. more than just black and white transmission/reflection, and at least one bit is addressed with a lesser number of grey levels. This permits a high number of greys to be achieved.
Again however the analogue levels achieved will be susceptible to temperature variations and a large number of spatial or temporal bits requires additional circuitry and faster addressing.
Zenithal bistable devices (ZBDs) are described in Bryan-Brown et al. “Grating Aligned Bistable Nematic Device”, Proc SID XXVIII, 5.3, pp 37-40 (1997) and U.S. Pat. No. 6,249,332. These use a surface alignment layer to give two stable states of a nematic liquid crystal material having either high or low surface tilt. The grating may be manufactured using either standard photolithographic methods or by embossing into a conformable layer on one of the inner surfaces of the display. When used opposite a conventional rubbed alignment surface the device may be latched between Hybrid Aligned Nematic (HAN) and Twisted Nematic (TN) configurations. These two configurations are shown in FIG. 2. The device is latched between states using electrical pulses of sufficient impulse, τV, where τ is the pulse duration and V its amplitude. In practice a display is addressed a line at a time using bipolar strobe, Vs, and data, Vd, pulses applied to the row and column electrodes simultaneously. Bipolar pulses are required to prevent unwanted latching effects due to a net DC across the pixel. The line-address-time is then equal to two time slots. Latching occurs on the trailing pulse of the high voltage resultant |Vs+Vd|. The leading pulse acts to both DC balance the waveform and to pole the ionic field before the latching pulse. The pixel remains unchanged with the opposite sign of data by ensuring that the low voltage resultant |Vs+Vd| is below the latching threshold.
Black and white ZBD displays are described in E. L. Wood et al. “Zenithal bistable device (ZBD) suitable for portable applications”, Proceedings of SID, 2000, v31, 11.2, p124-127 (2000) that show good front of screen performance combined with ultra-low power and rugged image storage. A 5 μm cell gap is used with manufacturing tolerances closer to those of conventional twisted nematic (TN) displays rather then Supertwisted nematic (STN) displays. These high tolerances allow complex displays to be fabricated readily using plastic substrates.
Greyscale has previously been achieved in a ZBD device by use of regions having different latching properties. A pixel is sub-divided into various regions, each having a different latching property. The sub-divisions are designed to give separately addressable areas using the using just one set of electrodes and drivers, each giving a discriminating operating window. Within this window the state of the pixel, and hence its transmission level, is insensitive to any variations of the latching threshold that may occur and may be termed ‘error-free’. Examples of multiple threshold techniques include varying the cell gap as shown in U.S. Pat. 4,712,877 or the applied field using electrode slits.
Alternatively the shape and alignment properties of the grating may be varied across a pixel, for example to give wide viewing angle and analogue greyscale Bryan-Brown et al. “Optimisation of the Zenithal Bistable Nematic Liquid Crystal Device” Proceedings of the 18th IDRC, Seoul, Korea, pp 1051-1053 (1998). For example each sub-pixel can be sub-divided into a number of areas with different latching thresholds. The fraction of the pixel that changes state, and hence its transmission, is then related to the applied electric signal. Alternatively International Patent Application WO02/08825 teaches that through appropriate grating design further stable configurations with different surface pre-tilts can be achieved to give a zenithal multistable device. Both of these approaches leads to error-free greyscale as shown in FIG. 3. Within the limits set by the partial latching regions of adjacent thresholds the transmission of error-free greys is independent of variations of the latching threshold.
It is an object of the present invention to provide a light modulating device, especially a bistable liquid crystal device, exhibiting greyscale which mitigates at least of the above mentioned disadvantages and/or allows greater levels of grey scale to be produced.